A Non-Secure Warning Tone (NSWT) is used in telephone calls to signal that a party of the call is non-trusted. The NSWT is applied to a voice signal of a telephone or voice call in order to be audible. A Multi-Level Secure (MLS) system is used for allowing secure communication between different security domains, for example secure and Non-Secure, and to use NSWTs to signal non-trusted parties in telephone communications. Particularly, MLS voice gateway can provide handling of voice calls between any combination of VoIP (Voice over Internet Protocol) and Circuit switched voice phones in different security domains.
The standard way of supporting NSWT is for the MLS system to issue the NSWT in-band. This method of supporting NSWT has critical limitations in VoIP systems, as it is essential that the NSWT is never lost, or degraded as to be unintelligible, which normally means the VoIP call must be kept at 64 kbps PCM, which has substantial bandwidth implications. It also means that the call must not be allowed to route out of the initial IP network into other attached IP networks, unless it can also be ensured that the NSWT will not be lost.
In the following, several of critical limitations issuance of the NSWT for the MLS system in-band are briefly explained:                It can never guarantee that the NSWT is always clearly audible on the VoIP phone.        It is highly reliant on the quality of the voice codec in order to minimize any possibility of the NSWT not being clearly audible. This will normally mean that it must use a high bandwidth codec, e.g. G.711/64 kbps, which clearly has substantial bandwidth implications.        It is essential that the routing of the VoIP call is controlled, and that the destination VoIP phone is on the same IP network, so as to minimize the possibility of the NSWT not being clearly audible.        It is affected by any degradation in the voice quality of the VoIP call, whether caused by multiple voice transcodes, or by packet delay or loss, as it is essential that the voice quality of the VoIP call is always keep high, so as to minimize the possibility of the NSWT not being clearly audible.        
A further problem is described in the following. In the past a NSWT has only been required to distinguish between two security domains, Secure and Non-secure, when operating in MLS mode. When it is necessary to support multiple security domains, e.g. in a Coalition, the single NSWT is no longer sufficient to ensure that the calling, or called, subscriber is fully informed as to what security domain the called, or calling, subscriber is in.
The standard way, when operating in MLS mode, to distinguish between multiple security domains is to just use a NSWT to distinguish between the multiple security domains, however this suffers from several key limitations:                It cannot guarantee that the security domain of a remote phone is clearly known to the called, or calling, subscriber.        It has a finite limit on the number of different security domains that can be supported, because as this number increases, this method of security warning rapidly becomes ambiguous and confusing.        